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The Light-Year-Long See-Saw Conundrum

-- | December 23, 2014

Question: A lunch friend and I are debating this question.

Essentially, consider a see-saw, a board or rod with a fulcrum in the middle. This rod is made of unbendable, nondestroyable fictional material. It will not bend.

Make this rod or board one light year long. One person sits on one end, the other sits on the other. Call them A and B. A pushes down and the board appears to raise up. I say that the speed at which the “information” of movement of the board is limited to the speed of light. Therefore, from A’s perspective, the movement is immediate. But from B’s perspective, she would not rise for an entire year, until the “information” of the movement of the board would finally reach her. In effect, the board would rise and fall at the speed of light. And the effect of up and down is not automatic depending on perspective. Am I totally off base? I thank you for considering the “See-Saw” conundrum. — Mike

Answer: Well, in fact, information, which in this case is a mechanical disturbance, will travel much slower than the speed of light in a rod or board. The compression wave that represents the mechanical force along the rod or board will travel at the speed of sound in the material that comprises the rod/board. This force travels through the rod or board by the electromagnetic interaction of the atoms in the rod or board. The speed of sound is much slower than the speed of light. For example, if the rod/board is made of steel it would take about 50,000 years for the mechanical force to reach observer B.

Jeff Mangum

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